Choosing wet cooling towers for rejecting waste heat and transferring to atmosphere are the best choices in many branches of industries including exothermic process. In these towers, hot water is sprayed on tower fills from top to bottom and ambient fresh air meet water on tower fills from bottom to top (in counter flow wet cooling tower) or laterally (in cross flow wet cooling towers), which lead to lower temperature of water.
During rejecting heat from water and transferring to atmosphere in this cycle, some portions of circulating water are vaporized and are discharged with air from tower, so water vapor plumes (fog) will form out of cooling tower. Water evaporation causes deduction in circulating water at 1 to 2 percent (or more) in the cycle, which known as the only problem of wet type cooling towers. (Various factors may influence circulating water amount like dispersion or splash of water, wind blowing and density variation of water (as an internal process in the tower), but up to 90% of water deduction is subjected due to evaporation).
Deduction in water quantity necessitates adding make up water to the system, so proving extra water, leads to increase in costs, deduction in regional water resources, and impact to environment and even in the case of water shortage the whole system will become inefficient. Furthermore, water deduction due to evaporation, increases water sediments, which results in some difficulties in pipes and pumps. In addition, exhaled fog creates visual pollution in environment, increase the risk of freezing tower adjacent roads, and can precipitate of ice and rain around tower during cold seasons.
Therefore, many efforts have been made to solve a problems regarding undesirable water evaporation, deduction or elimination of forming plums of water vapor (fog) produced by cooling towers. Utilizing hybrid (wet-dry) towers was considered as a solution, but high cost of commissioning and maintenance of such kind of towers prevents its wide application, so despite existence of fog in wet cooling towers, they still have the greatest application in cooling processes. Decreasing humidity of incoming fresh air into tower and pre-cooling of inlet warm water before entering to tower, were subjected as two other solutions but complexity, high cost and decreasing efficiency of towers have kept them away from universal applications.
As it is commonly used in many cases, the simplest method for solving problem of water deduction is utilization of drizzle eliminator suited on the route of discharged air that absorbs moisture content of exhausted air partially and restores to tower. Drizzle eliminators which are used in cooling towers nowadays, are usually installed horizontally either against vertical flow in wet cooling tower with counter flow, or against horizontal flow in wet cooling tower with cross flow. They are suitable for absorbing greater water drops (greater than 100 microns in diameter), while formed fog ordinarily has water droplets with diameter in the range of 2 up to 10 microns. Therefore, the current drizzle eliminators are not able to absorb majority of droplets and main percentages of moisture eliminated by them consist of splashed water around the tower.
A foresaid facts show that problems of cooling towers fog and consequently water deduction in water cycle have not solved effectively yet. Therefore, ground water sources, wells, and other water resources will be consumed to supply huge cooling towers of power plants, oil refineries, steel industries, and other industries with exothermic processes, which will increase the costs and will acquire dryness for the region.